Message-ID: <63f4ed716af37_d174a20880@willemb.c.googlers.com.notmuch>
Date:   Tue, 21 Feb 2023 11:12:33 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jiri Pirko <jiri@...nulli.us>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        pabeni@...hat.com, edumazet@...gle.com, mst@...hat.com,
        jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
        alvaro.karsz@...id-run.com, vmireyno@...vell.com, parav@...dia.com
Subject: Re: [patch net-next v2] net: virtio_net: implement exact header
 length guest feature

Jiri Pirko wrote:
> Tue, Feb 21, 2023 at 04:11:53PM CET, willemdebruijn.kernel@...il.com wrote:
> >Jiri Pirko wrote:
> >> From: Jiri Pirko <jiri@...dia.com>
> >> 
> >> Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when
> >> set implicates that the driver provides the exact size of the header.
> >> 
> >> Quoting the original virtio spec:
> >> "hdr_len is a hint to the device as to how much of the header needs to
> >>  be kept to copy into each packet"
> >> 
> >> "a hint" might not be clear for the reader what does it mean, if it is
> >> "maybe like that" of "exactly like that". This feature just makes it
> >> crystal clear and let the device count on the hdr_len being filled up
> >> by the exact length of header.
> >> 
> >> Also note the spec already has following note about hdr_len:
> >> "Due to various bugs in implementations, this field is not useful
> >>  as a guarantee of the transport header size."
> >> 
> >> Without this feature the device needs to parse the header in core
> >> data path handling. Accurate information helps the device to eliminate
> >> such header parsing and directly use the hardware accelerators
> >> for GSO operation.
> >> 
> >> virtio_net_hdr_from_skb() fills up hdr_len to skb_headlen(skb).
> >> The driver already complies to fill the correct value. Introduce the
> >> feature and advertise it.
> >> 
> >> Note that virtio spec also includes following note for device
> >> implementation:
> >> "Caution should be taken by the implementation so as to prevent
> >>  a malicious driver from attacking the device by setting
> >>  an incorrect hdr_len."
> >> 
> >> There is a plan to support this feature in our emulated device.
> >> A device of SolidRun offers this feature bit. They claim this feature
> >> will save the device a few cycles for every GSO packet.
> >> 
> >> Signed-off-by: Jiri Pirko <jiri@...dia.com>
> >> ---
> >> v1->v2:
> >> - extended patch description
> >
> >Is the expectation that in-kernel devices support this feature, and
> >if so how would it affect them? If I read the spec correctly, devices
> 
> Well, the tap driver actually trusts the hdr_len to be of correct header
> size nowadays.

tap_get_user performs basic bounds checking on the length passed.
 
> 
> >still need to be careful against malicious drivers, so cannot assume
> >much beyond what they do today (i.e., a hint).
> 
> Malicious how? There is upper limit of size in tap which is checked.
> I assume that for hw implementation, that would be the same.

A device cannot blindly trust a hdr_len passed from a driver. We have
had bugs in the kernel with this before, such as the one fixed in
commit 57031eb79490 ("packet: round up linear to header len").

> But anyway, this discussion would be rather part of the spec/device
> patch, don't you think?

I disagree. If it's not much effort to make a commit self-documenting
that is preferable. And if not, then an explicit reference to an
authoratitive external reference is preferable over "it is trivial to
look it up".
 
> 
> >
> >Might be good to point to the definition commit:
> >https://github.com/oasis-tcs/virtio-spec/commit/4f1981a1ff46b7aeb801c4c524ff76e93d9ce022
> 
> There were couple of fixes to the spec since then, that's why I didn't
> include it. It is trivial to look it up in the spec.

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